This invention relates to improvements in high pressure shaft seals to prevent the uncontrolled leakage of liquid along a rotating shaft, as for example the shaft of a rotary liquid pump.
Although the present invention is not limited to any particular application or field of use it was developed to meet the exacting requirements and extreme conditions encountered in primary coolant pumps for nuclear power plants where the existing prior art seals have proved inadequate or at least marginal in meeting performance requirements. Reliable and predictable performance of reactor coolant pump seals is essential to the safe operation and availability of nuclear power plants. An unscheduled plant shut down due to poor seal performance or seal failure can be extremely expensive to both the utility company and the seal supplier. Also, servicing of such seals is a very difficult and demanding job owing to the nuclear environment and limited accessibility in the reactor containment building.
Reactor coolant pump seals must be capable of operating under extreme conditions not encountered in other types of service. Steady state operating conditions under high pressure and high temperature present serious problems in themselves but reactor coolant pump seals must perform under widely changing conditions of pressure, temperature, and shaft position.
The normal operating pressure in a pressure water reactor is near 2200 psig and during start up of a plant the pressure can be as low as 300 psig.
Other excussions from normal pump operation as a result of support system malfunctions or failure also cause wide variations in the system operating pressure. Therefore the variations in seal operating pressure can be large in magnitude.
The same is true in regard to temperature. The normal operating temperature of the water in the reactor coolant loop is near 600.degree. F. Under normal operating conditions the shaft seals are cooled and pressure staged by water supplied from an auxiliary high pressure injection system. The nominal temperature of the water entering the seal area is 120.degree. F. In the event of a malfunction or failure of the injection system, the water in the seal cavity is replaced with water from the reactor coolant loop. Since this water is extremely hot it must be cooled in the pump heat exchanger prior to reaching the seal area. The temperature of the water in the seal area during this mode of operation is dependent upon total outflow from the seals (leakage plus staging flow) and heat exchanger performance characteristics. Therefore, the temperatures reached and the rate at which they change during any given loss of injection can be extremely variable.
A typical U.S. vertical reactor coolant pump is arranged with the shaft supported by three bearings, two of which are in the motor on the upper end of the shaft and one in the pump just above the centrifugal impeller. During operation the unbalanced radial load at the pump impeller, an inherent characteristic of a centrifugal pump, causes the shaft to bend between the pump bearing and motor bearings. The shaft is also displaced in the vertical direction (axial) during operation due to shaft end thrust (pressure loading) and thermal expansion. The magnitude and rate of displacement are dependent upon system operating conditions (system temperature, pressure, and pump flow rate). Maximum displacement (radial offset) occurs close to the position of the seal elements.
Since conventional (prior art) seals have proven only marginal in meeting the extreme and variable operating conditions experienced in reactor coolant pumps, the objectives of the present invention are to provide improved high pressure shaft seals capable of operating effectively under the described normal and transient operating conditions, to provide seals having a longer life than conventional seals, to provide seals having more predictable performance, to provide seals having improved reliability and maintainability, and to provide seals capable of easier assembly and faster installation.